There has been known a conventional traction control device that controls a slippage of a driving wheel of a vehicle by applying a regenerative torque (braking torque) to the driving wheel from an electric motor. In general, the device of this type makes the electric motor produce a constant regenerative torque when the driving wheel slips.
However, if the regenerative torque produced by the electric motor is constant, the regenerative torque easily becomes excessive or insufficient depending on the road surface condition. For example, in the case where the regenerative torque is relatively high, if the coefficient of friction of the road surface is relatively high, the slippage of the driving wheel tends to be abruptly eliminated, and thus, the acceleration of the vehicle tends to abruptly vary. On the other hand, in the case where the regenerative torque is relatively low, if the coefficient of friction of the road surface is relatively low, it tends to take a long time to eliminate the slippage of the driving wheel.
As the traction control device of this type, there has been known a traction control device disclosed in Japanese Patent Laid-Open No. 11-105688 (referred to as Patent Document 1, hereinafter). The traction control device disclosed in the Patent Document 1 controls a slippage of a driving wheel (brings a slip ratio of a driving wheel close to 0) using an electric motor coupled to the driving wheel and a friction brake device for the driving wheel when the slippage of the driving wheel is detected.